Liquid crystal display device

ABSTRACT

A liquid crystal display device includes a TFT substrate having a first alignment film and an opposing substrate having a second alignment film with liquid crystals sandwiched therebetween. One of the first and second alignment films, comprises a first polyimide produced via polyamide acid ester containing cyclobutane as a precursor and a second polyimide produced via polyamide acid as a precursor. The polyamide acid has a higher polarity than that of the polyamide acid ester. The one of the first and second alignment films is responsive to photo-alignment. A first side of the one of the first and second alignment films is adjacent to the liquid crystals, and a second side thereof is closer to one of the TFT substrate and the counter substrate than the first side. The first side contains more of the first polyimide and less of the second polyimide than the second side.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.14/493,893, filed Sep. 23, 2014, which is a continuation of U.S.application Ser. No. 14/167,477, filed Jan. 29, 2014, now U.S. Pat. No.8,854,582, which is a divisional of U.S. application Ser. No.13/028,311, filed Feb. 16, 2011, now U.S. Pat. No. 8,648,988, thecontents of which are incorporated herein by reference.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent ApplicationJP 2010-032443 filed on Feb. 17, 2010, the content of which is herebyincorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to a liquid crystal display device andparticularly to a liquid crystal display device including a liquidcrystal display panel in which alignment films are provided with thecapability of liquid crystal (LC) alignment control by lightirradiation.

BACKGROUND OF THE INVENTION

In a liquid crystal display (LCD) device, a TFT substrate over whichpixel electrodes and thin-film transistors, inter alia, are formed in amatrix and an opposing substrate over which color filters, inter alia,are formed in positions corresponding to the pixel electrodes in the TFTsubstrate are placed facing each other and liquid crystals aresandwiched between the TFT substrate and the opposing substrate. Animage is produced by controlling light transmissibility through liquidcrystal modules pixel by pixel.

Owing to the fact that LCD devices are flat and light, they are used inan increasing wide range of applications in various fields including alarge-screen display of TV and the like, mobile phones, Digital StillCamera (DSC), etc. Meanwhile, viewing angles are a problem specific toLCD devices. Viewing angles imply a phenomenon in which brightness andchromaticity change depending on when the screen is viewed squarely andwhen viewed from an oblique direction. An In Plane Switching (IPS) typeLCD in which liquid crystal molecules are moved by applying an electricfield in a horizontal direction offers superior viewing angleperformance.

As an LC alignment process applied for alignment films for use in an LCDdevice, that is, a method for providing the alignment films with thecapability of LC alignment control, rubbing is a conventionally usedmethod. The LC alignment process by this rubbing accomplishes alignmentof liquid crystals by rubbing the alignment films with a cloth. Analternative method for providing the alignment films with the capabilityof LC alignment control without touching the alignment films is called aphoto-alignment method. Because liquid crystals in the IPS type LCD donot need to have a pretilt angle, the photo-alignment method can beapplied to the IPS type LCD.

A photo-alignment process, namely, a photolytic LC alignment byirradiation of light such as, typically, ultraviolet light is disclosedin Japanese Published Unexamined Patent Application No. 2004-206091, inwhich the following are described about the photo-alignment process ofphotolytic LC alignment: (1) this process decreases the disturbance ofLC alignment due to a complex level difference configuration in a pixelregion; and (2) this process eliminates thin-film transistor breakdowncaused by static electricity generated in the LC alignment process byrubbing and a poor-quality display caused by the disturbance of LCalignment due to pilling of the rubbing cloth or dust attached theretoand also eliminates process complexity because of frequent rubbing clothreplacement required to obtain the capability of uniform LC alignmentcontrol.

In Japanese Published Unexamined Patent Application No. 2008-235900, atwo-layer alignment film structure is described, wherein an alignmentfilm capable of photo-alignment is formed in an upper layer and analignment film having a lower volume resistance than the upper layer isformed in a lower layer, thereby shortening the time in which anafterimage disappears. In Japanese Published Unexamined PatentApplication No. 2003-57147, a method for measuring azimuthal anchoringstrength which becomes a problem in photo-alignment is described.

SUMMARY OF THE INVENTION

In terms of providing the alignment films with the capability of LCalignment control, it is known that the alignment stability of thephoto-alignment process is generally lower than that of the rubbingprocess. Low alignment stability varies an initial LC alignmentdirection, resulting in a poor-quality display. Especially, in an LCDdevice using an IPS type LCD panel for which high alignment stability isrequired, low alignment stability tends to give rise to a power-qualitydisplay typified by afterimages.

In the photo-alignment process, a step of stretching and straighteningthe main chains of polymeric molecules as in the rubbing process doesnot exist in the LCD process. Instead, in the photo-alignment process,an alignment film made of a synthetic polymer typified by polyimide,irradiated by polarized light, is provided with uniaxial anisotropy in adirection perpendicular to the polarization direction, resulting fromthat the main chains of the polymer are broken in a direction parallelto the polarization direction. Liquid crystal molecules are alignedalong the orientation of long main chains that remained extendingstraight without being broken. If the length of the main chains isshort, it results in a decrease in the uniaxial anisotropy, which inturn weakens the interaction with liquid crystals. As a result, thealignment stability decreases and the above-mentioned afterimages areliable to occur.

Therefore, in order to improve the uniaxial anisotropy and the alignmentstability of an alignment film, an increase in the molecular weight ofthe alignment film is needed. As a solution for this, it is possible touse a photo-alignment film material obtained by imidization of polyamideacid ester. According to this solution, such polyamide acid estermaterial is not accompanied by a reaction of decomposition into diamineand acid anhydride during an imidization reaction, which would takeplace in a conventionally used polyamide acid material. Thus, thealignment film can be maintained to have a large molecular weight afterimidization and its alignment stability comparable to that provided bythe rubbing process can be obtained.

Because the polyamide acid ester material does not include a carboxylicacid in its chemical structure, it yields a higher voltage retentionrate of LCD as compared with a polyamide acid material and can ensure animprovement in long-term reliability.

For the meantime, as for LCD devices using photo-alignment, during longtime operation, the direction of initial alignment of liquid crystalswill offset from that direction initially determined when the LCD devicewas manufactured. Due to this, afterimages arise, which are called ACafterimages. It is found that these afterimages are generated becausethe azimuthal anchoring strength of alignment films is weak. Hence, theAC afterimages are irreversible and unrecoverable. The azimuthalanchoring strength means the strength that provides resistance againstthe offset of liquid crystals in an azimuthal direction from the initialalignment direction.

Meanwhile, afterimages also arise from charge accumulation in alignmentfilms. They are called DC afterimages. The DC afterimages are reversibleand disappear over time.

A problem of the present invention is to improve the azimuthal anchoringstrength of alignment films in the photo-alignment method and preventso-called AC afterimages from arising. An object of the presentinvention is to prevent so-called DC afterimages from arising or to makethe DC afterimages disappear quickly even if they arise.

The present invention overcomes the above-discussed problems and offerspractical means as will be outlined below. Specifically, an alignmentfilm for aligning liquid crystals is adapted to have a two-layerstructure including a photo-alignment film in an upper layer adjoiningliquid crystals and an alignment film with enhanced film strength in alower layer adjoining a substrate. The upper layer photo-alignment filmis formed of a precursor of polyamide acid ester containing 80% or morepolyamide acid ester including cyclobutane. The lower layer alignmentfilm with enhanced film strength is formed of a precursor of polyamideacid.

After drying and firing the alignment film thus including the twolayers, the film is irradiated with polarized ultraviolet light, so thatphoto-alignment of the photo-alignment film is performed. Thereafter,the alignment film is finished by heating the substrate irradiated withthe ultraviolet light.

The photo-alignment film is imidized at a rate of 50% or more. Thephoto-alignment film accounts for between 30% and 60% of the wholealignment film. The volume resistivity of the upper layerphoto-alignment film is larger than that of the lower layer alignmentfilm with enhanced film strength.

An alternative structure of the present invention is as follows: theupper layer photo-alignment film is formed of a precursor of polyamideacid containing 80% or more polyamide acid including cyclobutane, andthe lower layer alignment film with enhanced film strength is formed ofa precursor of polyamide acid not including cyclobutane. The fabricationprocess is the same as described above.

A further alternative structure of the present invention is as follows:the upper layer photo-alignment film is formed of a precursor ofpolyamide acid ester containing 80% or more polyamide acid esterincluding cyclobutane, and the lower layer alignment film with enhancedfilm strength is formed of a precursor of polyamide acid ester notincluding cyclobutane. The fabrication process is the same as describedabove.

According to the present invention, the alignment film has the two-layerstructure including the photo-alignment film adjoining liquid crystalsand the alignment film with enhanced film strength adjoining thesubstrate. Therefore, by way of photo-alignment, it is possible realizea liquid crystal display device in which the azimuthal anchoringstrength is strong and less afterimages appear after long timeoperation.

According to the present invention, after photo-alignment by performedby ultraviolet light irradiation, the substrate is heated at apredetermined temperature. Therefore, it is possible to enhance the (LC)anchoring strength of the alignment film, as degrading the mechanicalstrength of the alignment film is avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of an IPS type liquid crystaldisplay device;

FIG. 2 is a plan view of a pixel electrode in FIG. 1;

FIGS. 3A and 3B illustrate an alignment film structure according to thepresent invention;

FIGS. 4A and 4B illustrate the principle of a photo-alignment film;

FIGS. 5A and 5B are cross-sectional diagrams of an alignment film of thepresent invention;

FIG. 6 is a chemical formula of polyamide acid ester includingcyclobutane;

FIG. 7 is a chemical formula of polyamide acid including cyclobutane;

FIG. 8 is a process for forming the alignment film for photo-alignment;

FIG. 9 is a graph showing a relationship between azimuthal anchoringstrength and afterimage;

FIG. 10 is a graph showing a relationship among alignment filmstructure, process for forming the alignment film, and azimuthalanchoring strength;

FIG. 11 is a graph showing a relationship between process conditions forforming a photo-alignment film and azimuthal anchoring strength;

FIG. 12 shows a pattern for evaluating DC afterimages; and

FIG. 13 shows a result of DC afterimage evaluation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Subject matters of the present invention will be described in detail bymeans of the following exemplary embodiments.

First Embodiment

FIG. 1 is a cross-sectional diagram showing a structure in a displayregion of an IPS type liquid crystal display device. A variety ofelectrode structures for IPS type liquid crystal display devices areproposed and put in practical use. The structure shown in FIG. 1 iswidely used now. In simple terms, over a common electrode 108 formed ina flat monolithic form, a comb-shaped pixel electrode 110 is formed withan insulation layer intervening therebetween. By rotating liquidcrystals 301 depending on a voltage between the pixel electrode 110 andthe common electrode 108, light transmissibility through a liquidcrystal layer 300 is controlled pixel by pixel and an image is thusproduced. The structure in FIG. 1 will be described in detail below.While the present invention is described by taking the structure in FIG.1 as an example, the invention can be applied to an IPS type liquidcrystal display device having a structure other than that shown in FIG.1.

In FIG. 1, a gate electrode 101 is formed on the top of a TFT substrate100 made of glass. The gate electrode 101 is formed in the same layer asfor a scan line. The gate electrode 101 is formed of an AlNd alloy and aMoCr alloy layered over the AlNd alloy.

A gate insulation film 102 covering the gate electrode 101 is formed ofSiN. On the top of the gate insulation film 102, a semiconductor layer103 is formed of a-Si film in a position opposed to the gate electrode101. The a-Si film is formed by plasma CVD. The a-Si film defines achannel portion of a TFT, and a source electrode 104 and a drainelectrode 105 are formed over the a-Si film, sandwiching the channelportion therebetween. In addition, an n+Si layer, not shown, is formedbetween the a-Si film and the source electrode 104 or the drainelectrode 105. The n+Si layer is formed for providing an ohmic contactbetween the semiconductor layer and the source electrode 104 or thedrain electrode 105.

The source electrode 104 overlaps a part of an image signal line and thedrain electrode 105 is connected to the pixel electrode 110. Both thesource electrode 104 and the drain electrode 105 are formed together inthe same layer. In the present embodiment, the source electrode 104 orthe drain electrode 105 is formed of a MoCr alloy. If it is desired todecrease the electrical resistance of the source electrode 104 or thedrain electrode 105, an electrode structure in which, for example, anAlNd alloy is sandwiched between MoCr alloys is used.

An inorganic passivation film 106 covering the TFT is formed of SiN. Theinorganic passivation film 106 protects the TFT, particularly, itschannel portion against impurities 401. Over the inorganic passivationfilm 106, an organic passivation film 107 is formed. Since the organicpassivation film 107 protects the TFT and also acts to planarize thesurface, it is formed thick. Its thickness ranges from 1 μm to 4 μm.

As the material of the organic passivation film 107, a photosensitiveacryl resin, silicon resin, or polyimide resin, inter alia, is used. Inthe organic passivation film 107, a through hole 111 needs to be formedin a location to connect the pixel electrode 110 and the drain electrode105. Because the organic passivation film 107 is photosensitive, thethrough hole 111 can be formed by exposing the organic passivation film107 itself to light and through development without using a photoresist.

On the top of the organic passivation film 107, the common electrode 108is formed. The common electrode 108 is formed by sputtering ITO (IndiumTin Oxide), which makes a transparent, electrically conductive film,over the display region. That is, the common electrode 108 is formed ina planar form. After forming the common electrode 108 over the surfaceby sputtering, the common electrode 108 is removed by etching only inthe portion of the through hole 111 to provide electrical conductionbetween the pixel electrode 110 and the drain electrode 105.

An upper insulation film 109 covering the common electrode 108 is formedof SiN. After the upper insulation film 109 is formed, the through hole111 is formed by etching. By etching the inorganic passivation film 106,using the upper insulation film 109 as a resist, the through hole 111 isformed. Then, an ITO film, which becomes the pixel electrode 110,covering the upper insulation film 109 and the through hole 111, isformed by sputtering the ITO. The pixel electrode 110 is formed bypatterning the ITO film deposited by sputtering. The ITO film whichbecomes the pixel electrode 110 is also deposited on the walls of thedrain hole 111. This makes electrical conduction between the drainelectrode 105 extending from the TFT and the pixel electrode 110 in thethrough hole 111 and an image signal is thus supplied to the pixelelectrode 110.

FIG. 2 shows one example of the pixel electrode 110. The pixel electrodeis a comb-shaped electrode. Slits 112 are defined between each combteeth. The planar common electrode is formed under the pixel electrode110. When an image signal is applied to the pixel electrode 110, liquidcrystals 301 are rotated by lines of electric force generated betweenthe pixel electrode 110 and the common electrode 108 and passing throughthe slits 112. Thereby, light passing through the liquid crystal layer300 is controlled to produce an image.

FIG. 1 is also intended to explain this aspect. Gaps between each combteeth of the comb-shaped electrode correspond to the slits 112 as shownin FIG. 1. A constant voltage is applied to the common electrode 108 anda voltage of an image signal is applied to the pixel electrode 110. Whenthe voltage is applied to the pixel electrode 110, as shown in FIG. 1,lines of electric force are generated to rotate liquid crystals 301 inthe direction of the lines of electric force and control thetransmission of light from a backlight. Due to the fact that thetransmission of light from the backlight is controlled pixel by pixel,an image is produced.

In the example of FIG. 1, the common electrode 108 formed in the planarform is placed on the top of the organic passivation film 107 and thecomb-shaped electrode 110 is placed on the top of the upper insulationfilm 109. Conversely to this, however, there may be a case where a pixelelectrode 110 formed in a planar form is placed on the top of theorganic passivation film 107 and a comb-shaped common electrode 108 isplaced on the top of the upper insulation film 109.

Over the pixel electrode 110, an alignment film 113 is formed to alignthe liquid crystals 301. In the present invention, the alignment film113 has a two-layer structure including a photo-alignment film 1131adjoining the liquid crystal layer 300 and an alignment film withenhanced film strength 1132 formed underlying the photo-alignment film1131. The structure of the alignment film 113 will be described indetail later.

In FIG. 1, an opposing substrate 200 is placed across the liquid crystallayer 300. Inside the opposing substrate 200, color filters 201 areformed. Red, green, and blue color filters 201 are formed for each pixelto produce a color image. Between each color filter 201, a black matrix202 is formed to improve an image contrast. The black matrix 202 alsoacts as a light shielding film of the TFT and prevents a photo currentbeing flowing in the TFT.

An overcoat film 203 covering the color filters 201 and black matrixes202 is formed. Because the surface of the color filters 201 and blackmatrixes 202 is uneven, the surface is planarized by the overcoat film203.

Over the overcoat film 203, an alignment film 113 is formed to determinean initial alignment of liquid crystals. The alignment film 113 of theopposing substrate also has a two-layer structure including aphoto-alignment film 1131 adjoining the liquid crystal layer 300 and analignment film with a lower resistance 1132 formed underlying thephoto-alignment film 1131. Because of IPS that is shown in FIG. 1, thecommon electrode 108 is formed in the TFT substrate 100, not in theopposing substrate 200.

As shown in FIG. 1, in IPS, an electrically conductive film is notformed inside the opposing electrode 200. This makes the potential ofthe opposing electrode 200 unstable. In addition, an externalelectromagnetic noise intrudes the liquid crystal layer 300 and has aninfluence on an image. To eliminate such a problem, a surfaceelectrically conductive film 210 is formed over the outside of theopposing electrode 200. The surface electrically conductive film 210 isformed by sputtering ITO, which makes a transparent, electricallyconductive film.

FIGS. 3A and 3B schematically illustrate an alignment film 113 accordingto the present invention. FIG. 3A is a transparent plane view of thealignment film 113 and FIG. 3B is a cross-sectional perspective viewthereof. The alignment film 113 of the present invention has a two-layerstructure including an upper photo-alignment film 1131 adjoining theliquid crystal layer and a lower alignment film with enhanced filmstrength 1132.

The molecular formula of polyamide acid ester is represented by chemicalformula (1).

In chemical formula (1), R1 is, individually, an alkyl group having acarbon number from 1 to 8, R2 is, individually, a hydrogen atom,fluorine atom, chlorine atom, bromine atom, phenyl group, alkyl grouphaving a carbon number from 1 to 6, alkoxy group having a carbon numberfrom 1 to 6, vinyl group (—(CH2)m-CH═CH2, m=0, 1, 2) or acetyl group(—(CH2)m-C≡CH, m=0, 1, 2), and Ar is an aromatic compound.

Chemical formula (1) is polyamide acid ester including cyclobutane, butthere is also polyamide acid ester not including cyclobutane. However,because what is capable of photo-alignment is polyamide acid esterincluding cyclobutane, polyamide acid ester including cyclobutane shouldaccount for 80% or more of the alignment film material.

The molecular formula of polyamide acid in FIG. 3A is represented bychemical formula (2). Chemical formula (2) is an exemplary structure ofpolyamide acid not including cyclobutane.

In chemical formula (2), R2 is, individually, a hydrogen atom, fluorineatom, chlorine atom, bromine atom, phenyl group, alkyl group having acarbon number from 1 to 6, alkoxy group having a carbon number from 1 to6, vinyl group (—(CH2)m-CH═CH2, m=0, 1, 2) or acetyl group(—(CH2)m-C≡CH, m=0, 1, 2) and Ar is an aromatic compound.

Unlike chemical formula (1), chemical formula (2) does not includecyclobutane. Because the alignment film with enhanced film strength doesnot need to perform photo-alignment, it is unnecessary for this film toinclude cyclobutane. Conversely, polyamide acid represented by chemicalformula (2) is not susceptible to ultraviolet light, because nocyclobutane exists in it. In addition, a difference between chemicalformula (1) and chemical formula (2) lies in that R1 existing inchemical formula (1) representing polyamide acid ester is replaced by Hin chemical formula (2).

Since FIG. 3A is a transparent view, a photolytic polymer 10 in thephoto-alignment film and a non-photolytic polymer 11 in the alignmentfilm with enhanced film strength are visible in a transparent fashion.In FIG. 3B, the alignment film 113 is formed over the pixel electrode110 or organic passivation film 107 in FIG. 1. In FIG. 3B, the alignmentfilm 113 is formed over the pixel electrode 110. The thickness t1 of theupper photo-alignment film 1131 is about 50 nm and the thickness t2 ofthe lower alignment film with enhanced film strength 1132 is about 50nm. The boundary between the photo-alignment film 1131 and the alignmentfilm with enhanced film strength 1132 is indefinite and, therefore, itis drawn by a dotted line.

FIGS. 4A and 4B schematically illustrate the principle for aligningliquid crystals by the photo-alignment film 1131. In FIGS. 4A and 4B,the alignment film with enhanced film strength 1132 is omitted. FIG. 4Ashows a state in which the photo-alignment film 1131 has been deposited.The photo-alignment film 1131 is formed of the photolytic polymer 10.

The photo-alignment film 1131 shown in FIG. 4A is irradiated withultraviolet light polarized in a horizontal direction, e.g., at anenergy of 6 J/cm². In the photo-alignment film 1131, then, thephotolytic polymer 10 in the polarization direction of the polarizedultraviolet light is broken by the ultraviolet light, as is shown inFIG. 4B. That is, breaks 15 are made by the ultraviolet light along thepolarization direction of the ultraviolet light. In consequence, liquidcrystal modules are aligned in a direction of arrow A in FIG. 4B.

If the main chains of the photolytic polymer 10 are short, as shown inFIGS. 4A and 4B, it results in a decrease in the uniaxial anisotropy ofthe alignment film, which in turn weakens the interaction with liquidcrystals. In consequence, the alignment performance decreases. Hence, itis desirable that the photolytic polymer 10 extends as long as possiblein the direction of arrow A in FIG. 4B even after the photo-alignment.In other words, an increase in the molecular weight of the alignmentfilm 113 is needed in order to improve the uniaxial anisotropy and thealignment stability of the alignment film 113.

The molecular weight of the alignment film 113 can be evaluated in termsof a number average molecular weight. Given that polymers with diversemolecular weights exist in the alignment film 113, the number averagemolecular weight is an average molecular weight among the polymers. Inthe photo-alignment film 1131, a number average molecular weight of 5000or more is required to obtain sufficient alignment stability.

In order to achieve the photo-alignment film 1131 with such a largenumber average molecular weight, imidized polyamide acid ester can beused. The structure of polyamide acid ester is as given previously inchemical formula (1).

Polyamide acid ester is characterized by R1 in chemical formula (1). Inthe polyamide acid ester, R1 is CnH2n−1, where n is 1 or more. If thepolyamide acid ester is used as a precursor of the photo-alignment film1131, it is not accompanied by a reaction of decomposition into diamineand acid anhydride during an imidization reaction, which would takeplace in a conventionally used polyamide acid material. Thus, thephoto-alignment film can be maintained to have a large molecular weightafter imidization and its alignment stability comparable to thatprovided by the rubbing process can be obtained.

However, the photo-alignment film suffers from a decrease in the filmstrength, because the main chains are broken in a particular direction.Study efforts made by the present inventors have revealed thatdegradation in the azimuthal anchoring strength of an alignment filmrelates to the mechanical strength of the alignment film. Thus, bymaking the alignment film of the two-layer structure and by disposingthe alignment film with enhanced film strength in the lower layer andthe photo-alignment film in the upper layer, the film strength of thewhole alignment film is increased, which is effective for improving theazimuthal anchoring strength of the alignment film.

FIGS. 5A and 5B are cross-sectional diagrams to schematically illustratea way of forming the two-layer alignment film. Forming the alignmentfilm 113 of the two-layer structure can be accomplished without anadditional process for forming the alignment film 113. That is, as shownin FIG. 5A, a mixture material of the photolytic polymer 10 and thepolymer 11 for forming the alignment film with enhanced film strength isdeposited onto the substrate. Then, one substance that will more readilysettle on the substrate is deposited in the lower layer and the othersubstance is deposited in the upper layer by a leveling effect, as shownin FIG. 5B; so-called phase separation takes place.

In this embodiment for depositing the two-layer alignment film, what iscalled the substrate is the ITO film from which the pixel electrode 110is formed or the organic passivation film 107. In comparison withpolyamide acid ester, polyamide acid has a higher polarity (largersurface energy) and will more readily settle on the ITO film or theorganic passivation film 107. Hence, polyamide acid always makes thelower layer. In the present invention, since the number averagemolecular weight of the alignment film with enhanced film strengthformed of polyamide acid is larger than that of the photo-alignment filmformed of polyamide acid ester, phase separation can take place moreeasily in addition to the polarity or surface energy effect. Of the twolayers of alignment film, the photo-alignment film 1131 accounts forbetween 30% and 60% of the whole alignment film. One reason for this isdisposing the photo-alignment film with a sufficient thickness in theupper layer and another reason is that phase separation is easy to takeplace after depositing the two-layer alignment film.

By heating the thus formed resin film at about 200° C., the alignmentfilm is imidized. Imidization is performed for both polyamide acid 1132in the lower layer and polyamide acid ester 1131 in the upper layer atthe same time. Therefore, it is possible to form the two-layer alignmentfilm 113 through the same process as for forming a one-layer alignmentfilm 113.

In order to stabilize LC alignment performance, the photo-alignment film1131 in the upper layer needs to be imidized at a high rate, as it isrequired to increase the photolysis efficiency of the photolytic polymer10. This is because a photolysis reaction is hard to take place, unlessthe photo-alignment film is well imidized. Since polyamide acid ester isgenerally hard to imidize, an imidization accelerator may be added as anadditive for aiding the imidization. The photo-alignment film 1131should be imidized at a rate of 50% or more, more preferably, 70% ormore. The remaining is polyamide acid ester existing as a precursor inthe photo-alignment film 1131.

On the other hand, because the alignment film with enhanced filmstrength 1132 in the lower layer has no relation to liquid crystalalignment performance, a rate at which it should be imidized does notneed to be specified particularly. That is, a condition for imidizationmay be set with regard to the imidization of polyamide acid ester in theupper layer.

The boundary between the upper and lower layers of the alignment film isindefinite. This boundary is denoted by a dotted line in FIG. 5B. InFIG. 5B, the photo-alignment film 1131 in the upper layer is composed ofpolyamide acid ester; particularly, polyamide acid ester includingcyclobutane, as is shown in FIG. 6, accounts for 80% or more of thewhole. That is, in the polyamide acid ester, cyclobutane is decomposedby polarized ultraviolet light and this cyclobutane decomposition yieldsphoto-alignment performance. Thus, it is requisite that the proportionof polyamide acid ester including cyclobutane is considerably large.

On the other hand, the alignment film with enhanced film strength 1132in the lower layer is formed of polyamide acid; it is desirable thatpolyamide acid including cyclobutane which is shown in FIG. 7 does notexist therein. Instead, polyamide acid not including cyclobutane, asrepresented by chemical formula (2), is used. That is, this is becauseit will be expedient that the alignment film with enhanced film strengthin the lower layer is not decomposed by ultraviolet light in order tomaintain the mechanical strength of the alignment film as the whole evenafter photo-alignment.

As explained above, the two-layer alignment film is characterized inthat it is formed such that the photo-alignment film 1131 in the upperlayer includes a considerable amount of cyclobutane, whereas thealignment film with enhanced film strength 1132 in the lower layer doesnot include cyclobutane. In other words, the photo-alignment film 1131adjoining liquid crystals includes a considerable amount of cyclobutane,whereas the alignment film with enhanced film strength 1132 adjoiningthe pixel electrode or organic passivation film, substantially, does notinclude cyclobutane.

As above, even if the upper layer is formed of polyamide acid esterincluding a considerable amount of cyclobutane, as a precursor of thealignment film 113, and the lower layer is formed of polyamide acid notincluding cyclobutane, as a precursor, the mechanical strength of thewhole alignment film may degrade depending on post processing forcarrying out photo-alignment.

FIG. 8 is a flowchart illustrating a process for photo-alignment. InFIG. 8, after depositing an alignment film, the film is dried. Until thefilm has been dried, the alignment film is phase-separated into twolayers of the photo-alignment film 1131 and the alignment film withenhanced film strength 1132.

Then, the alignment film is imidized by firing (burning) it. After that,photo-alignment is performed. Conventionally, the substrate, while beingheated at about 200° C., is irradiated with linearly polarizedultraviolet light for photo-alignment. However, during ultraviolet lightirradiation that is performed, while the substrate is heated, photolysistakes place by the ultraviolet light also in the lower layer alignmentfilm with enhanced film strength and results in a decrease in the filmhardness.

In contrast, the present invention adopts a process in which thesubstrate is irradiated with polarized ultraviolet light without beingheated and, after that, the substrate is heated, as shown in aright-hand section surrounded by a dotted line in FIG. 8. By carryingout this process, the lower layer alignment film with enhanced filmstrength can be prevented being subjected to photolysis by theultraviolet light and the required film strength of the alignment filmas the whole can be maintained.

Owing to the structure as described above, the initial azimuthalanchoring strength of the alignment film can be improved. FIG. 9 shows arelationship between azimuthal anchoring strength and a rate of changeof afterimage brightness. As for a method for measuring azimuthalanchoring strength, we used the method described in Japanese PublishedUnexamined Patent Application No. 2003-57147. In FIG. 9, the abscissaindicates azimuthal anchoring strength in units of 10⁻³ J/m² and theordinate indicates a rate of change of afterimage brightness. A rate ofchange of afterimage brightness is determined as follows. For example, achecker flag pattern, as is shown in FIG. 12, is displayed for 12 hoursand, then, the display returns to a gray flat halftone. A gray level ofthe halftone is 64/256. At this time, a brightness ratio between a lightcell and a dark cell in the checker flag is a rate of change ofafterimage brightness.

In FIG. 9, if a rate of change of afterimage brightness is 1% or less,afterimage may be regarded to be invisible. In FIG. 9, in order to makeafterimage invisible, azimuthal anchoring strength is required to be3.5×10⁻³ J/m² or more.

FIG. 10 is a table showing a result of comparison of azimuthal anchoringstrengths depending on alignment film structure and process forphoto-alignment. In FIG. 10, the above comparison is made between theone-layer structure of alignment film, as used conventionally, and thetwo-layer structure of alignment film. With regard to the process forphoto-alignment, the above comparison is made between the process inwhich substrate heating and ultraviolet light irradiation are performedat the same time and the process in which the substrate is heated afterultraviolet light irradiation.

As can be seen in FIG. 10, a condition satisfying a target value, i.e.,azimuthal anchoring strength of 3.5×10⁻³ J/m² or more, is only the casewhere the alignment film has the two-layer structure and the process inwhich the substrate is heated after ultraviolet light irradiation isused. That is, by using the present invention, it is possible to realizea liquid crystal display device that, substantially, solves anafterimage problem in the photo-alignment method.

As explained above, it is possible to obtain an alignment film in whicha countermeasure against afterimages was taken by the process conditionfor photo-alignment. FIG. 11 is a table showing a result of evaluatingprocess tolerance to avoid afterimages. In FIG. 11, azimuthal anchoringstrengths are compared, based on the following parameters: ultravioletlight (UV) irradiance level, heating temperature after ultraviolet lightirradiation, interval from ultraviolet light irradiation until substrateheating.

In FIG. 11, the parameter of ultraviolet light irradiance level isenergy obtained by integrating the energies of all wavelengths from 230to 330 nm. The ultraviolet light, particularly, polarized ultravioletlight is used. A ratio (extinction ratio) between the intensity ofultraviolet light in the polarization direction and that in thedirection perpendicular to the polarization direction is not less than15:1 in the range of the wavelengths from 230 to 330 nm.

In FIG. 11, condition A defines that the ultraviolet light irradiancelevel is from 2000 to 5000 mJ/cm², the heating temperature afterultraviolet light irradiation is not less than 230° C., and the intervalafter ultraviolet light irradiation until substrate heating is withinone hour. Condition B defines that the ultraviolet light irradiancelevel is from 1000 to 7000 mJ/cm², the heating temperature afterultraviolet light irradiation is not less than 200° C., and the intervalafter ultraviolet light irradiation until substrate heating is within 24hours. Condition C defines that the ultraviolet light irradiance levelis from 500 to 8000 mJ/cm², the heating temperature after ultravioletlight irradiation is not less than 150° C., and the interval afterultraviolet light irradiation until substrate heating is within 168hours.

In FIG. 11, condition A can provide the largest azimuthal anchoringstrength, i.e., an azimuthal anchoring strength of 4.2. Next, conditionB provides an azimuthal anchoring strength of 4.0 and condition Cprovides an azimuthal anchoring strength of 3.5. It is desirable thatphoto-alignment is performed according to condition A. However,photo-alignment according to condition A may be impossible because ofrequirements of a fabrication line. It is required to ensure particularprocess conditions for photo-alignment complying with at least the levelof condition C or more.

In the foregoing description of the embodiment, the alignment film isdiscussed which uses polyamide acid ester, 80 percent or more of whichis polyamide acid ester including cyclobutane, as the upper layerphoto-alignment film and uses polyamide acid not including cyclobutaneas the lower layer alignment film with enhanced film strength. However,the present invention is not so limited. Even with an alignment filmwhich uses polyamide acid ester, 80 percent or more of which ispolyamide acid ester including cyclobutane, in the upper layer and usespolyamide acid ester not including cyclobutane in the lower layer, it ispossible to achieve the desired effect by heating the substrate afterultraviolet light irradiation as the process for photo-alignment.

Moreover, even with an alignment film which uses polyamide acid, 80percent or more of which is polyamide acid including cyclobutane, in theupper layer and uses polyamide acid not including cyclobutane in thelower layer, it is possible to achieve the desired effect by heating thesubstrate after ultraviolet light irradiation as the process forphoto-alignment.

Second Embodiment

In the first embodiment, how the alignment film structure and theprocess for photo-alignment contribute to an advantageous effect againstAC afterimages is mainly discussed. The structure of the presentinvention can achieve the effect against so-called DC afterimages,besides AC afterimages.

DC afterimages are a phenomenon resulting from charge accumulation incertain portions of the alignment film. Hence, DC afterimages are areversible phenomenon, because they disappear when charges dissipate. Inorder to avoid DC afterimages, it is conceivable that the alignment filmis adapted to have a structure that facilitates fast dissipation ofcharges accumulated in the alignment film or a structure that primarilyprevents charges from being accumulated in the alignment film.

First, descriptions are provided for the structure that facilitates fastdissipation of accumulated charges. We evaluated DC afterimages asfollows. That is, the checker flag pattern made up of 8×8 white andblack cells, as shown in FIG. 12, is displayed for 12 hours and, then,the display returns to a gray flat halftone. A gray level of thehalftone is 64/256. At 10 minutes after the return to the halftone, ifthe checker flag pattern can be recognized, the test result is FAIL; ifthe pattern cannot be recognized, the test result is PASS.

As described in the first embodiment, the alignment film of the presentinvention includes the upper layer photo-alignment film and the lowerlayer alignment film with enhanced film strength. In the secondembodiment, the alignment film with enhanced film strength is adapted tohave a volume resistance of 10¹⁴ Ωcm or less, preferable, 10¹³ Ωcm orless. That is, this volume resistance is made smaller by one digit thanthe volume resistance of the upper layer photo-alignment film. Thisvolume resistance may be that obtained when the alignment film isirradiated with light from the backlight. Thereby, charges charged inthe alignment film are discharged soon.

FIG. 13 shows a evaluation result from comparing DC afterimages for thealignment film of the two-layer structure as above and for aconventional one-layer alignment film. In FIG. 13, the abscissaindicates time elapsed after the return to the gray flat halftone andthe ordinate indicates an afterimage level. On the ordinate, RRindicates a state that the checker flag pattern is visible well at thereturn to the halftone, which is FAIL. R indicates a state that thechecker flag pattern is visible, but vaguely at the return to thehalftone.

In FIG. 13, curve A is a DC afterimage characteristic when the alignmentfilm according to the present invention is used. Curve B is an exampleof a DC afterimage characteristic when a single layer photo-alignmentfilm is only used as the alignment film.

Even though the afterimage level is R at the return to the halftone, ifthe afterimage disappears in a short time, it can be considered to bepractically no program. In the case of the single layer photo-alignmentfilm, the level R at the return to the halftone persists long and,practically, a problem remains. On the other hand, for the alignmentfilm 113 of the two-layer structure according to the present invention,DC afterimage rapidly attenuates and completely disappears at about 17minutes after the return to the halftone.

As explained above, a large difference between the single layerphoto-alignment film and the photo-alignment film of the presentinvention is that DC afterimage persists long in the case of the singlelayer photo-alignment film, whereas DC afterimage rapidly attenuatesthrough the use of the alignment film of the present invention. In FIG.13, according to the present invention, DC afterimage becomes 25% orless at 10 minutes after the return to the halftone, whereas DCafterimage is 90% or more at the corresponding time in the case of thesingle layer photo-alignment film.

An alternative method as a countermeasure against DC afterimages is toadapt the alignment film to have a structure that prevents charges frombeing accumulated in the alignment film, even if a certain pattern isdisplayed for a long time. This can be accomplished by increasing thevolume resistance of the alignment film extremely. In order toaccomplish this, in the present invention, the alignment film of thetwo-layer structure is used, wherein the photo-alignment film is formedin the upper layer and the alignment film with enhanced film strength isformed in the lower layer. In this structure, the volume resistivity ofthe lower layer is made larger than that of the upper layer. The upperlayer photo-alignment film has a small degree of freedom in varying itsvolume resistance, restricted by its photo-alignment performance. On theother hand, the lower layer alignment film with enhanced film strengthcan have a large degree of freedom in varying its volume resistance.

If the volume resistance of the alignment film with enhanced filmstrength is made larger than 10¹⁵ Ωcm, the electrical resistance of thealignment film as the whole becomes larger, thereby impeding chargesfrom being accumulated in the alignment film and the passivation film.Since the volume resistance of polyamide acid ester as thephoto-alignment film is as large as about 10¹⁵ 0 cm, by making thevolume resistance of the lower layer alignment film with enhanced filmstrength larger than 10¹⁵ 0 cm, charges are further impeded from beingaccumulated in the alignment film.

The structure of the two-layer alignment film described in the first andsecond embodiments is digested as below. In the structure including thephoto-alignment film in the upper layer and the alignment film withenhanced film strength in the lower layer, polyamide acid ester is usedin the upper layer and polyamide acid is used in the lower layer,wherein the upper layer contains 80% or more polyamide acid esterincluding cyclobutane. The upper layer is imidized at a rate of 50% ormore. In the structure as above, the volume resistivity of the lowerlayer is made smaller than that of the upper layer in order to reduce DCafterimages.

As an example of another structure of the two-layer alignment film,polyamide acid is used in the upper layer and polyamide acid is used inthe lower layer, wherein the upper layer contains 80% or more polyamideacid including cyclobutane. The upper layer is imidized at a rate of 50%or more. In the structure as above, the volume resistivity of the lowerlayer is made smaller than that of the upper layer in order to reduce DCafterimages.

As an example of yet another structure of the two-layer alignment film,polyamide acid ester is used in the upper layer and polyamide acid esteris used in the lower layer, wherein the upper layer contains 80% or morepolyamide acid including cyclobutane. The upper layer is imidized at arate of 50% or more. In the structure as above, the volume resistivityof the lower layer is made larger than that of the upper layer in orderto prevent DC afterimages.

While the foregoing description concerns the alignment film 113 in theTFT substrate 100, the same holds true for the alignment film 113 in theopposing substrate 200. The alignment film 113 in the opposing substrate200 is formed over the overcoat film 203. In this case also, thenon-photolytic polymer 11 from which the alignment film with enhancedfilm strength 1132 is formed will more readily settle on the overcoatfilm 203. Consequently, the alignment film with enhanced film strength1132 is formed contiguous to the overcoat film 203 and thephoto-alignment film 1131 is formed on the top of the lower layeralignment film. Moreover, because the number average molecular weight ofthe alignment film with enhanced film strength is larger than that ofthe photo-alignment film, phase separation is easier to take place.

What is claimed is:
 1. A method for fabricating a liquid crystal displaydevice including a TFT substrate having a photo-alignment film formedover a pixel electrode and a common electrode; an opposing substratewhich faces the TFT substrate; and a liquid crystal layer sandwichedbetween the TFT substrate and the opposing substrate; wherein the methodcomprising steps of: forming a mixture material that is a material ofthe photo-alignment film on the TFT substrate, imidizing the mixturematerial so that a precursor contained in the mixture material isimidized, irradiating a polyimide formed from the imidized precursorwith ultraviolet light for photo-alignment without heating; and heatingthe polyimide to form the alignment film after the irradiation.
 2. Themethod for fabricating a liquid crystal display device according toclaim 1, wherein an irradiance level of the ultraviolet light is from2000 to 5000 mJ/cm2, a temperature at which the TFT substrate and theopposing substrate are heated after the ultraviolet light irradiation isnot less than 230° C., and an interval after the ultraviolet lightirradiation until heating the TFT substrate and the opposing substrateis within one hour.
 3. The method for fabricating a liquid crystaldisplay device according to claim 1, wherein an irradiance level of theultraviolet light is from 1000 to 7000 mJ/cm2, a temperature at whichthe TFT substrate and the opposing substrate are heated after theultraviolet light irradiation is not less than 200%, and an intervalafter the ultraviolet light irradiation until heating the TFT substrateand the opposing substrate is within 24 hours.
 4. The method forfabricating a liquid crystal display device according to claim 1,wherein an irradiance level of the ultraviolet light is from 500 to 8000mJ/cm2, a temperature at which the TFT substrate and the opposingsubstrate are heated after the ultraviolet light irradiation is not lessthan 150%, and an interval after the ultraviolet light irradiation untilheating the TFT substrate and the opposing substrate is within 168hours.
 5. The method for fabricating a liquid crystal display deviceaccording to claim 1, further comprising: forming color filters on theopposing substrate.
 6. The method for fabricating a liquid crystaldisplay device according to claim 1, wherein the precursor is polyamideacid ester.
 7. The method for fabricating a liquid crystal displaydevice according to claim 1, wherein the precursor is polyamide acid. 8.The method for fabricating a liquid crystal display device according toclaim 7, wherein the precursor includes cyclobutane.
 9. The method forfabricating 3 liquid crystal display device according to claim 1,wherein the alignment film is separated into two layers.